1. Field of the Invention
The present invention relates to a communication apparatus and a communication method, and, more particularly, to a communication apparatus and a communication method which employ a data communication frame formed by combining a plurality of packets.
2. Description of the Background Art
In general, when incorporating packets (e.g., IP packets) provided from an upper layer or packets (e.g., ethernet frames) provided from another protocol inside abridge into a frame in the Medium Access Control (MAC) layer of a data communication system, a header containing control information, such as a destination address, a source address, a frame type, etc., is always assigned to the head of each packet. From the viewpoint of the upper layer, however, this header information is overhead, and may be a factor for decreasing packet transmission efficiency.
Methods for reducing overhead to reduce the decrease in the packet transmission efficiency are proposed in Japanese Laid-Open Patent Publication No. 7-123118 and Japanese Laid-Open Patent Publication No. 2003-69642. In these methods, as illustrated in FIG. 13, headers (H1 to H4) specific to respective packets (P1 to P4) are not provided, but instead, a plurality of packets are combined together and then a single header (H) is assigned to a group of packets (i.e., a frame) thus obtained.
When the above-described methods of combining a plurality of packets for transmission are applied to a communication system which employs, as a medium, an air (wireless), a power line, or the like, the following problem arises.
In communication systems which employ, as a medium, an air (wireless), a power line, or the like, it is difficult to accurately grasp whether another communication apparatus is currently transmitting data, and it is impossible to determine the end of a frame. Therefore, for example, in the case where header portions are transmitted at a low rate and data portions are transmitted in a highly efficient manner by employing a modulation system (e.g., Discrete Multi Tone (DMT)) specific to communication apparatuses that communicate with each other, data collision may occur.
One conceivable method to solve this problem is to write within a header a period of time required (or the number of symbols to be transmitted) for completing the transmission of a frame containing a plurality of packets, by adopting a conventional method of adding to a header portion the information of a period of time (in microseconds) required (or the number of symbols to be transmitted) for completing the transmission of a frame and thus reporting the information to a communication apparatus on the receiving end. This conventional method is disclosed in IEEE Std 802.11, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”.
In order to implement this method, it is necessary to calculate the total transmission time for a frame containing combined packets (or the total number of symbols to be transmitted) before transmitting the frame. However, it is impossible to calculate in advance the total transmission time for a frame (or the total number of symbols to be transmitted) in the case of a system in which an optimal value of a modulation parameter is dynamically selected for each pair of communication apparatuses that communicate with each other in accordance with transmission path fluctuation (e.g., a system that employs DMT in multicarrier transmission, in which transmission path fluctuation is regularly examined, and each time transmission path fluctuation is examined, an optimal modulation parameter is selected), and in the case where packets to be combined together may have various data lengths.
Therefore, it is necessary to repeatedly perform, when transmitting a frame, a process of obtaining a current modulation parameter and the data length of a current packet stored in a buffer and, after adding the packet to the frame, carrying out calculation with respect to combining of packets. Therefore, as the number of packets to be combined together increases, the time required for calculating the total transmission time (or the total number of symbols to be transmitted) becomes longer. As a result, the interval between the completion of the transmission of a frame and the start of the transmission of the next frame becomes long (i.e., the interframe gap becomes large). In other words, the transmission efficiency of the communication system as a whole becomes reduced, despite the intention to improve the transmission efficiency by combining packets.
In addition, in order to avoid conflict between a plurality of communication apparatuses, an interframe gap specific to a system should be set. At this time, the value of the interframe gap specific to the system should be the period of time required for completing calculation with respect to the maximum number of packets that can be combined together, even in the case where only a small number of packets are actually to be combined together. This is an inefficient operation.